1. Field of the Invention
The present invention relates to a fuel reforming apparatus for generating a hydrogen-rich reformed gas to be supplied as a power generating fuel for a fuel cell, and particularly relates to an evaporator for forming the fuel gas used for the reformation reaction.
2. Background Art
For example, a solid polymer electrolyte-type fuel cell system is a power generating system comprising a fuel cell body formed by stacking laminates, each formed by a solid polymer electrolyte film sandwiched by an anode catalystic electrode and a cathode catalystic electrode. The power is generated by supplying the hydrogen-rich reformed gas formed by the fuel reforming apparatus and air (oxygen) to produce an electrochemical reaction. Recently, this solid polymer electrolyte-type fuel cell has become widely used as the driving power supply for automotive vehicles.
The fuel reforming apparatus mainly comprises an evaporator for evaporating a hydrocarbon (for example, methanol) fuel-water mixture, which includes hydrocarbons (for example, methanol) and water, a reformer for producing a hydrogen-rich reformed gas by introducing a fuel vapor through a reforming catalytic layer, and a CO removing device for selectively removing carbon monoxide in the reformed gas by introducing the fuel vapor through the selective oxidation catalytic layer.
In general, it is known that a boiler is used for generating a vapor, and it is also known that a stacking plate-type, shell-type or tube-type heat exchangers may be used for generating vapor. In addition, since the boiler has a capacity for producing a necessary amount of vapor and has an ability to store a considerable amount of vapor, the boiler can supply the necessary amount of vapor when necessary. However, when not necessary, the vapor valve must be closed. That is, the boiler must be provided with pressure tightness to be able to withstand the internal pressure rise at the time of closing the vapor room, so that the boiler inevitably become large, and thus, the boiler is not suitable for the automotive vehicles.
The plate type heat exchanger is advantageous in its high thermal efficiency. However, since it is necessary for the plate type heat exchanger to provide enough space for the passage for passing the combustion gas in order to avoid the pressure loss of the combustion gas from being compressed in the passage, the heat exchanger must be large.
In addition, since the plate-type heat exchanger is designed such that the vapor is sequentially obtained by vaporizing the liquid phase stored in the inside of the heat exchanger, it is not possible to stop vaporization when vapor is not necessary. If a valve is installed at the outlet of the plate type heat exchanger, the heat exchanger is required to be pressure tight and thus it becomes large.
Although the shell and tube-type heat exchanger has the advantageous feature that it is possible for the combustion gas as the heat source to flow with low pressure loss, it is necessary to improve the efficiency of the heat exchange for generating vapor. Furthermore, it is necessary to stabilize the temperature fluctuations because the vapor temperature fluctuates depending upon the load (the vapor temperature is high at low loads, but low at high loads).
The shell and tube heat exchanger is designed such that the combustion gas is passed through the tube, while a jet of the fuel is emitted on the outside surface of the tube. If the fuel falls to the bottom of the shell as liquid droplets due to scattering of the fuel during atomization, the hydrocarbon (for example, methanol) fuel-water mixture is collected at the bottom, making it difficult to stop the vaporization and to suspend the vapor supply when the vapor is not necessary. Furthermore, since the atomized fuel is directly blown onto the tube, the tube is subjected to severe thermal stress due to the cyclic contacts with combustion gas and hydrocarbon (for example, methanol) fuel-water mixture.
As shown above, the conventional boilers and heat exchangers do not satisfy the requirements for evaporators for on-vehicle fuel cell systems, i.e., small size, improved vaporization response (elimination of the evaporation residue by instantaneous evaporation), and high efficiency.
It is desired for the evaporator of the on-vehicle fuel cell to provide a long service life by constituting a structure capable of reducing the thermal stress during operation.
It is therefore an objective of the present invention to provide a small evaporator for an on-vehicle fuel cell capable of instantaneously evaporating the hydrocarbon (for example, methanol) fuel-water mixture, stabilizing the vapor temperature with respect to fluctuations of the load, and improving the durability by reducing the thermal stress.
In order to achieve the above objectives, the present invention provides a fuel reformer, used for generating a hydrogen-rich reformed gas by reforming a fuel vapor generated by evaporating a hydrocarbon (for example, methanol) fuel-water mixture into the fuel vapor by an evaporator, wherein said evaporator comprises: an evaporation chamber for generating the fuel vapor by blowing the atomized hydrocarbon (for example, methanol) fuel-water mixture on heating medium tubes, in which a combustion gas is circulated; and a heating chamber for heating the fuel vapor by circulating the combustion gas output from a vapor tube, in which the fuel vapor is conducted.
In the evaporation chamber, the hydrocarbon (for example, methanol) fuel-water mixture is atomized and blown on the heating medium tube, in which the combustion gas circulates, so that the hydrocarbon (for example, methanol) fuel-water mixture is instantaneously evaporated without leaving a residue. Thus, it is not only possible to form a necessary amount of fuel vapor instantaneously, but it is also possible to cut the fuel vapor effectively. Instantaneous evaporation of the hydrocarbon (for example, methanol) fuel-water mixture without leaving a residue prevents a rise of the internal pressure in the evaporation chamber and thus does not require a pressure tight structure.
In the heating chamber, the fuel vapor generated in the evaporation chamber is heated passing through the vapor tubes arranged in the combustion gas atmosphere, the fuel vapor can be heated homogeneously and the temperature of the heated gas can be stabilized, in contrast to the heating by sending the fuel vapor around the heating medium tubes in which the combustion gas passes. Thus, it becomes possible to supply a fuel vapor by maintaining a temperature of the fuel vapor suitable for reforming, even if the amount of fuel vapor fluctuates due to load fluctuations.
The present invention provides a construction such that the combustion gas output from the heating medium tubes is forwarded to the heating chamber through the guide path along the floor of the evaporation chamber, even when the hydrocarbon (for example, methanol) fuel-water mixture drops on the floor of the evaporation chamber, the liquid drops will be evaporated due to the heated floor of the evaporation chamber, which allows a more efficient cut of the fuel vapor.
The present invention provides an auxiliary evaporator for generating the fuel vapor by use of the combustion gas output from the evaporator, which makes it possible to compensate for a shortage of the fuel vapor when the necessary amount of the fuel vapor increases suddenly, so that the present fuel reforming system provides a superior response to the fluctuating demands for fuel vapor. In addition, since the combustion gas exhausted from the evaporation chamber is used as the heat source for the auxiliary evaporator, a new heat source is not required.
Furthermore, the present invention provides a structure in which the fuel vapor generated in the auxiliary evaporator is introduced into the evaporation chamber and is sent to the reformer after passing through the heating chamber, and it is possible to supply the fuel vapor including the original fuel vapor generated in the auxiliary evaporator after heating at a temperature suitable for reforming, even when the load requires a sudden increase of the original fuel vapor.